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[Help-smalltalk] GC/grey pages question

From: Holger Hans Peter Freyther
Subject: [Help-smalltalk] GC/grey pages question
Date: Sat, 27 Sep 2014 18:01:33 +0200
User-agent: Mutt/1.5.23 (2014-03-12)

Dear Paolo,

I was trying to understand the garbage collection better (e.g. to
determine if adding a write-barrier is a pain, if direct pointers
will be feasible at some point).

I thought the concept was:

* We mprotect READ_ONLY the old space
* When an object from old space starts to point to the new space
  it will be modified and the modification will trigger a SIGSEGV
* The SIGSEGV handling will add the page to the greylist.

* At some point the GC will scan the grey list, mark the objects
  and move them to the old space too.
* The entry from the greylist will be removed and the page will
  be marked as read-only again.

The reality seems to be a bit different.

* We don't seem to mprotect the oldspace but the OOP table? Is
this correct or am I misguided?

* We slowly/never remove entries from the greylist? E.g. even
after doing ObjectMemory compact the greylist is not empty? Is
this a bug?

kind regards

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